The use of so-called "automatic processors" is conventional for developing imagewise exposed silver halide elements such as X-ray films. The exposed films are fed into the processor from a safelight area, i.e. one in which the level of light is reduced below that which exposes the film further, and is transported through various chambers containing the developer, the fixer and the water washes. At the end of the processor, the film is dried and exits into normal light as a finished product ready for use. This is illustrated in U.S. Pat. No. 3,545,971 "Rapid Processing of Photographic X-ray Film" (1966). This system is rapid and convenient and large numbers of films can be routinely handled in this manner over long periods of time without deleterious effects to the finished image. Of course, the processing solutions become exhausted by the passage of the exposed silver halide film and replenishment must be made to account for ingredient loss. Additionally, the processing fluids, especially the developer solution, are degraded aerobically by contact with air and anerobically when simply left for long periods of inaction.
When large amounts of film are being processed (high throughput), exhaustion of the developer is due almost entirely to development of the silver halide image. The development reaction as shown in Mason's "Photographic Processing Chemistry" Focal Press (1966) page 74, is: EQU 2AgBr+H.sub.2 Q+Na.sub.2 SO.sub.3 .fwdarw.2Ag+HBr+NaBr+HQSO.sub.3 Na
Thus, some materials, i.e., hydroquinone and sulfite are lost while bromide, acid, and hydroquinone monosulfonate are formed. Also, not shown by this reaction, antifoggant may be lost. Current developer replenishers are formulated based on this reaction as follows: The replenisher is made higher in pH than developer and contains no bromide, so as to offset the acid and bromide released by the development reaction. The other ingredients are set at concentrations which allow for the expected losses, the hydroquinone being offset by the antifoggants, without substantially affecting the sensitometry. In this situation a replenishment rate can be conveniently calculated based on the approximate area of film fed into the processor.
The other situation presents a more difficult problem because when only small amounts of film are processed (low throughput), the developer deteriorates primarily from air oxidation and thermal reaction, and not from development reaction; Mason's "Photographic Processing Chemistry" supra at page 73: EQU H.sub.2 Q+2Na.sub.2 SO.sub.3 +O.sub.2 .fwdarw.HQSO.sub.3 Na+NaOH+Na.sub.2 SO.sub.4.
That is, hydroquinone and sulfite are lost but not bromide. Also, pH increases, not decreases as in the development reaction. Also, since film passage activates the replenishment, low throughput decreases replenishment.
Current practice is to compensate for low throughput by increasing replenishment rate significantly, e.g., as much as two-fold. This, however, can actually accelerate the problems caused by low throughput processing and cause sensitometric instability. It does so because the replenisher, which is being added, in excess, has higher pH than the developer and no bromide. The effect of this is to (1) maintain the increasing pH of the developer caused by oxidation, and (2) dilute the bromide content to levels so low as to cause sensitometric instability, reflected mainly in fog and speed.
The present invention provides a replenisher formulation which is aimed at compensating for developer changes caused by nonuse, and not by the development reaction. Thus, the replenisher is characterized by a pH lower than that of the developer, and it contains the maximum amount of bromide consistent with acceptable sensitometry and contains enough antifoggant to minimize loss during use. Now when replenishment occurs, pH will remain constant or slightly decrease, the bromide will remain essentially constant, and the other changes will compensate as in state of the art formulations. That is, the developer hydroquinone, sulfite, and antifoggant losses suffered during periods of nonuse are not sufficiently great to cause sensitometric instability. Before this occurs replenishment occurs, restoring these ingredients to concentrations closer to their initial values. Thus, by maintaining the developer composition more constant, the developer can maintain a useful lifetime equivalent to the high throughput processors. Because of the optimal developer/replenisher balance for low throughput, the developer is stabilized with less replenishment than the conventional developers when attempting to process these same low throughput conditions.
It is accordingly an object of this invention to provide an improved developer/replenisher system for developing exposed silver halide photographic film. A further object is to provide a developer/replenisher system particularly useful for machine processing of X-ray film when low throughput of film is practised.